Hereinafter, the term “scrambling code” in this specification is equivalent with the term “long code” in the claim.
Multicarrier transmission system such as multicarrier CDMA (Code Division Multiple Access) or OFDM (Orthogonal Frequency Division Multiplexing) modulates a source information signal with a plurality of subcarriers and inserts guard intervals (GIs) to modulated signal in order to reduce waveform distortion due to multiple delayed-paths.
FIG. 1 shows a conventional transmitter 1000 for Multicarrier CDMA (refers to ‘MC-CDMA’, hereinafter) systems. The transmitter 1000 provides a plurality of data channel generation circuit 100. In each data channel generation circuit 100, a transmission data generator 101 generates a transmission data sequence. An encoder 102 encodes the generated data sequence. A data modulator 103 modulates the encoded data sequence. The modulated data symbol sequence is multiplexed with a pilot by a multiplexer 104. A serial-parallel converter 105 converts the multiplexed symbol sequence into a sequence of N/SF symbols allocated along a frequency axis.
A copier 106 copies each of the sequence of N/SF symbols so that a total length of the copies is equal to a period of the short code. A short code generator 107 generates the short code uniquely assigned for each subscriber. The copies allocated along the frequency axis are multiplied with the short code by multipliers 108, respectively.
A combiner 109 combines the outputs from each data channel generation circuit 100.1-100.x and outputs N combined signals. The N combined symbols are respectively input to other multipliers 111. A scrambling code generator 110 generates a scrambling code (a long code) and outputs to respective multipliers 111. Each multiplier 111 multiplies the respective combined signals with the scrambling code, to provide N symbols scrambled by the scrambling code.
An IFFT unit 113 carries out inverse fast Fourier transform to the N combined symbols and transforms into an orthogonal multicarrier signal. A GI adder 114 inserts guard intervals to the transformed multicarrier signal. This multicarrier signal is transmitted as a wireless signal.
A receiver for MC-CDMA system receives the multicarrier signal transmitted from the transmitter and removes the GIs from the multicarrier signal. The receiver further carries out FFT (Fast Fourier Transform) to separate the received multicarrier signal into the N subcarrier components and recovers the original data sequence. Before carrying out FFT, the receiver must detect FFT timing.
To detect the FFT timing, the OFDM employs a technique by using a correlation characteristic of a guard interval. This technique is disclosed in “A Simultaneous Estimation of Symbol Synchronization and Frequency Offset of Multicarrier Modulation Signals” by Mori, Okada, Hara, Komaki, and Morinaga in IEICE Technical Reelement RCS95-70, pp. 9-16 (1995-09). Another technique is proposed that transmits a timing detection signal twice, and on a receiver side, detects a correlation between two symbols. This technique is disclosed in “Study on Synchronization Method for High-Speed Wireless LAN OFDM System” by Onizawa, Mizoguchi, Kumagai, Takanashi, and Morikura in IEICE Technical Reelement RCS97-210, pp. 137-142 (1998-01).
In MC-CDMA systems, each subscriber is identified according to a short code allocated to each subscriber. Therefore, multiple subscribers may simultaneously carry out communications in the same frequency band.
A mobile communication system employing the MC-CDMA must employ scrambling codes to identify respective base stations. Therefore, the receiver for the multicarrier CDMA system must be able to identify a scrambling code as well as to detect a FFT timing. Accordingly, each mobile station must detect correlations in connection with all scrambling codes prepared by the system and detect a scrambling code related to a signal from a base station to which the mobile station must be connected. For flexible allocation of scrambling codes for each base station, the system must prepare several hundreds of scrambling codes. This raises a problem that a mobile station must spend long time to detect a proper scrambling code before starting communication with a target base station. Studies on the multicarrier CDMA, however, are mostly related to the evaluation of link levels, and none are related to the identification of scrambling codes.